Method of manufacturing semiconductor devices



Jan. 19, 1965 G. B. FINN, JR., ETAI.. 3,166,449

METHOD OF MANUFACTURING SEMICONDUCTOR DEVICES Original Filed May 2, 1957./Z ig) IN V EN TURS, l (76011005. P22121. Ji: BY and (oberP/'SOMSUnited States Patent O 3,166,449 METHUD F MANUFACTURENG SEMI- CUNDUCTRDEVICES George ll. Finn, Jr., Los Angeles, Calif., and Robert C.

Parsons, Cincinnati, Ohio, assignors to Sarlies Tarzian, Inc.,Bloomington, Ind., a corporation of Indiana Original application May 2,1957, Ser. No. 656,621, now Patent No. 3,051,878, dated Ang. 2S, 1962.Divided and this application Jan. 2, 1962, Ser. No. 163,610 5 Claims.(Cl. 148-177) The present invention relates to semiconductor devices andprincipally tok high current capacity P-N junction silicon rectifierswhich are suitable for use as power rectitlers. Specifically, thisapplication is a division of a prior application, Serial No. 656,621,filed May 2, 1957, which issued as U.S. Patent No. 3,051,878, and isassigned to the same assignee as the present invention.

P-N junction silicon rectifiers have heretofore been used in low powerapplications, and because of their high forward-to-reverse currentresistivity ratio it would be desirable also to use them for relativelyhigh power applications where, for example, selenium rectitiers are nowemployed. Since silicon rectifiers heretofore known operatesatisfactorily only when the silicon is maintained at relatively lowtemperatures as, for example, below 200 C., means must be provided fordissipating the heat generated at the junction during use and heat sinkshaving large masses have been employed for cooling the silicon. In orderto operate efficiently, the heat sinks should be attached to the siliconthrough a good thermoconductive connection. A `suitable solder may beused for this purpose.

Although silicon rectifiers are very small in size as compared toselenium or other rectifiers of comparable current and power capacity,their cost of manufacture hasY been so much higher than that of seleniumrectifiers or the like that the silicon rectiiiers have been used,principally, only for `special applicationswhere space or high qualityoperation is an important factor. Therefore, in order to make the costof silicon rectifiers competitive with other power rectifiers of similarcurrent ratings, their manufacutring cost must be greatly decreased.This is accomplished in accordance with the present invention bysimultaneously forming the P-N junction in a silicon wafer and solderingsuitable terminal members and heat sinks to opposite sides of thejunction in a single heating operation. Moreover, this junction formingand fusion operation is carried out at temperatures which are criticaland with materials not having critical compositions of numerouselements. Y

It is a principal object of this invention to provide a new and improvedrectifier and a method of manufacturing it.

Another object of this invention is to provide a silicon rectifier inwhich all solder bonds and a P-N junction are formed during a singlehigh temperature operation.

A further object of this invention is to provide an improved method offorming a P-N junction in a silicon crystal wafter.

Brieiiy, the above and further objects are realized in accordance withthe present invention by providing a rectifier which includes a set oftransition members which are interposed between the silicon crystal andthe terminal v members, which transition members are formed of tantalumor niobium. These transition members are soldered to opposite sides ofthe crystal wafer during an alloying operation in which the P-N junctionis formed in the silicon crystal. One of the transition members has across-sectional area which is substantially less than that of the wafer,and the other transition member has a cross-sectional area which isapproximately equal to or 3,156,449 .Patented Jan. 19, 1965 ICC tivesizes of elements are conventional and insure a large surface leakagepath between the transition members.

ln accordance with an important aspect of the present invention, the P-Njunction is formed by an alloying process in which the side of the wafernearest the large area transition member is alloyed to form thejunction. This manner of forming the junction provides a P-N junctionwhich extends across the entire cross-sectional area of the crystal. 1fthe junction is formed from the opposite or small area side, as hasformerly been done in all silicon diodes, the area of the P-N junctionwhich is formed is appreciably less than the cross-sectional area of thewafer and, therefore, the current capacity and the power rating of awafer of given dimensions is lower.

Further objects and advantages of the present invention may be had fromthe following detailed description taken in connection with the singlefigure of the drawing which is a cross-sectional View of a siliconrectifier assembly prior to a heating operation in which the partsthereof are fused together and a P-N ljunction is formed in a siliconwafer.

Referring now to the drawing wherein is illustrated an assembly 1t) ofthe principal components of a silicon rectifier embodying the presentinvention, the stack of elements is shown prior to the formation of aP-N junction in the single crystal silicon wafer 11 and the fusion ofthe individual components of the rectifier together.

ln addition to the silicon wafer 11, which is of the N- type, theassembly 10 comprises an end contact and heat sink 12, a resilientcontact 13, and a pair of transition members 14 and 15 which arerespectively disposed between the silicon wafer 11 and the contacts 12and 13. The silicon wafer 11 is preferably very thin and, as a result,is also very fragile. Therefore, in order to prevent damage to the waferduring the formation of a junction therein and also during use of thecompleted rectifier, transition members having a thermal coefficient ofexpansion closely approximating that of silicon are interposed betweenthe contacts 12 and 13 and the silicon wafer. In addition, it isimportant that the transition members be good conductors of both heatand electricity if satisfactory operation of the diode is to beachieved. Moreover, and in accordance with an important aspect of thisinvention more fully described hereinafter, the transition membersshould be formed of a metal which does not react with silicon at therelatively high temperatures at which the junction in the silicon waferis formed and which may be satisfactorily bonded to the silicon at thistemperature. We have found that tantalum, niobium and base alloys ofeach, satisfactorily meet all of these requirements. While there areother metals such, for example, as molybdenum which have thermalcoefiicients of expansion similar to that of silicon at relatively lowtemperatures and which actually have higher thermal and electricalcoefficients of conductivity, such metals react with silicon attemperatures below l000 C. Accordingly, if the transition members areformed of these other metals, the junction forming and fusion operationsmust be carried out at less than the temperatures at which suchreactions occur. However, by using tantalum or niobium for thetransition members, temperatures of the order of 1l00 C. may be used,and as a result, diodes having lower forward resistance may be producedin an economical manner, close control of the solder ingredients and ofthe junction forming and fusion temperatures being unnecessary. Y

In accordance with the present invention, a P-N junction is formed byyalloying a portion of an N-type silicon wafer with aluminum. Thealloying is accomplished at relatively high temperatures and fusion ofthe various elements of the rectifier together is carried out in thissame operation.

In order to form the P-N junction in the wafer 11 by an alloyingprocess, a thin sheet of aluminum 17, which preferably comprises analuminum-gallium alloy including a small amount of gallium such, forexample, as one to five percent by weight, is positioned adjacent to oneface of the wafer 11 between the wafer and the transition member 11i.Moreover, in order to provide a good ohmic and mechanical connectionbetween the alloyed portion of the wafer 11 and the transition member14, a thin sheet of substantially pure tin 20 is interposed between thealuminum sheet 1'7 and the transition member 14. When, therefore, theassembly is heated to a temperature of the order of 1100 C. during thejunction forming alloying operation and its thereafter cooled, thealloyed portion of the wafer 11 is fused to the transition member 14.

In order to provide a good mechanical and purely ohmic connectionbetween the smaller transition member and the wafer 11, a thin sheet ordot of tin or lead 21 having a small percentage such, for example, asonehalf percent by weight of a donor impurity such, for example, asantimony, is disposed between the wafer 11 and the transition member 15.Moreover, during the alloying process when the sheet 21 melts, theadjoining surface of the wafer 11 is maintained N positive by the donorimpurity so that any acceptor impurity such, for example, as aluminumvapor which may be present during the alloying operation, cannot effecta junction at this side of the wafer.

During the high temperature alloying operation the entire stack ofelements including the contacts 12 and 13 may be fused together or, inthe alternative, only the transition members need be fused to the wafer11.

If the entire assembly including the terminal members 12 and 13 is to befused during the alloying process, thin sheets of lead 23 `and 24 arepositioned between the transition members 14 and 15 and the contacts 12and 13. The lead thus provides a solder for bonding the members 14 and15 to the contacts 12 and 13. Tin is unsuitable for soldering thetransition members to the terminal members during the alloyingoperation, because at the high temperatures used for alloying the tindissolves or diifuses into the copper terminal. As a result, nointerface is effected and the members do not bond together when the unitis cooled.

Under certain circumstances it is preferable to form the junction in onehigh temperature operation and thereafter to solder the transitionmembers to the terminal members in a lower temperature operation.Therefore, in order to facilitate the making of a good bond between thetransition and terminal members during the latter operation, the outerfaces of the transition members are tinned during the junction formingoperation. Tin may be used for this purpose since the copper is notpresent during the high temperature junction forming operation and tindoes not diffuse or dissolve into tantalum or niobium at thetemperatures involved. On the other hand, if the rectifier assembly 10is fused together in the alloying operation without the contacts 12 and13, these contacts to be later soldered thereto, the sheets 23 and 24are formed of tin. The reason for using lead instead of tin when theentire unit is fused during the `alloying operationl is that at the hightemperatures involved in the alloying operation the tin would diffuseinto the copper members 12 and 13 so that no bond would be effectedbetween the contacts 12 and 13 and the transition members 14 and 1S.When, however, the terminals 12 and 13 are not bonded to the rectifierduring the alloying operation, tin may be used and is preferred overlead since it has a lower melting point and provides a better solder.

During the alloying and fusion operation, the assembly 10 is placed inan alloying oven or furnace which is maintained at a temperature ofbetween 850 and 1100 C. At this temperature excellent wetting of the tinor lead to the tantalum and copper occurs. At lower temeratures, such asthose which must be employed where the transition members are formed ofmolybdenum or the like, considerably poorer wetting and thus a poorerbond and an ohmic connection of higher resistance results. While thejunction is being formed, the components of the assembly are maintainedin compressed relationship by any suitable means such as, for example, acarbon cylinder (not shown) positioned over the upper end of the contact13, the weight of the cylinder functioning to force the various parts ofthe assembly onto the contact 12. After the alloying operation iscompleted, which takes about one to thirty minutes depending upon thethermal inertia of the system, the assembly is removed from the oven andas the temperature thereof decreases the molten parts solidify and theentire assembly is fused together.

Following the fusion and alloying operation the Vrectier may besubjected to an etching process to increase its back voltage rating.This etching process removes any extraneous and conductive materialwhich may be present on the exposed surface of the junction. 1n thosecases in which the terminal members are not fused to the transitionmembers during the alloying operation, the rectifier units are notetched until after the terminal members have been soldered thereto.

The process carried out during the etching operation is as follows:

(l) The rectifier is immersed in a boiling etching solution ofapproximately 10% NaOH for about ten minutes. Alternatively, similarsolutions of KOH or LiOH may be used as the etchant.

(2) The assembly is then rinsed as in water, which is preferablyboiling, and which has been deionized so as to have a conductivity ofnot less than 0.010 mmho per centimeter.

(3) After the rinsing operation, the assembly is placed in a watersolution of l-10% nitric acid for about one minute to neutralize thehydroxide and to remove any metals which may have deposited across thesurface of the junction.

Upon completion of the etching process, the unit is then finished inaccordance with the following process:

(1) The neutralized assembly may then be treated with a solution ofsoda-ash as described in copending application, Serial No. 654,905,filed in the name of George Eannarino, and assigned to the same assigneeas the present invention.

(2) The unit is then washed in distilled water having a conductivity ofnot less than .050 mmho per centimeter.

(3) The distilled water is then blown off the rectifier by, for example,a jet of steaml or hot nitrogen. It is important that the water be blownoff the rectifier rather than evaporated from it since evaporation may,in some cases, deposit small amounts of impurities on the junction whichwould decrease the surface leakage resistance.

(4) The surface of the junction is then coated with silicone Varnishhaving a coating thickness of less than .002 inch. Dow Corning #997 issatisfactory for this purpose. This coating protects the junction fromdust and moisture and prevents galvanic action between the variousmetals of the junction.

(5) rThe varnished unit is then baked at 180 C. for eight or more hoursto cure the varnish and dry out the unit.

(6) While the unit is still warm, a resilient silicone rubber such as,for example, Dow Corning #6126, is troweled onto the unit over the loopin the spring terminal member 13.

(7) The unit is then maintained at a temperature of C. for four hours inorder to cure the silicone rubber. The function of the silicone rubberis described in a copending application, Serial No. 656,622, filed inadsense the name of George Eannarino and George B. Finn, Ir., andassigned to the same assignee as the present invention. While particularembodiments of the invention have Vbeen shown, it will be understood, ofcourse, that it is not desired that'the invention be limited theretosince modifications may be made, and it is, therefore, con-- templatedby the appended claims to'cover any such modiications as fall within thetrue spirit and scope of f the invention.

Vgroup consistingv or tantalum and niobium, a thin sheet consistingessentially of tin, a thin sheet ot an alloy consisting essentially ofaluminum and 1 to 5% by weight voi gallium, av thin wafer of an N-typesilicon, a thinl sheet of a solder consisting essentially of tin and adonor impurity, and another transition member formed of one of the groupconsisting of tantalum and niobium maintaining each of the components ofsaid stack in contact with the adjacent components; heating said stackto a temperature of between 850 and 1100 C.; maintaining said stack atsaid temperature for a predetermined time interval, and thereaftervreducing the temperature of said stack. K p

2. A method of making a semiconductor diode which comprisessuperimposing uponV one another in the order specified, a copperterminal member, a thin sheet of lead, a transition member termed of oneof the group consistvingV oftantalum and niobium, a thin sheetconsisting esconsisting essentially of tin and a donor impurity, an-`other transition member -formed of one of the group consisting oftantalum and niobium, a thin sheetof lead, and

another copper terminal member, maintaining the components of said stackin Contact with the adjacent components and heating said stack to atemperature of between 856 and 11G0 C. maintaining said stack at saidtemperature for a predetermined time interval and thereafter reducingthe temperature of said stack tofroom temperature. v

3. A method of making a semiconductor diode which comprisessuperimposing upon one another in the order speciiied, a transitionmember formed of one of the group consisting of tantalum and niobium, a`thin sheet consisting essentially of tin, a thin sheet of an alloyconsisting essentially of aluminum and 1 to 5% by weight of gaii lium,and a thin wafer of an N-type silicon, maintaining the components ofsaid stack in contact with the adjacent Y components and heating saidstack tol a temperature in,`

the vicinity of lOl'GLC., maintaining said stack at said vten'iperaturefor a predetermine'dtime interval and there- References Cited in thefile of this patent UNITED STATES PATENTS 2,763,822 Frola et al. Sept.18, 1956 2,801,375 Losco ylluly 30, 1957 2,917,686v Boyer et al Dec. l5,1959 2,922,092 GaZZara et al. Ian. 19, 1960 2,945,285 Jacobs July 19,196()

1. A METHOD OF MAKING A SEMICONDUCTOR DIODE WHICH COMPRISES SUPERIMPOSING UPON ONE ANOTHER IN THE FOLLOWIN ORDER, A TRANSITION MEMBER FORMED OF ONE OF THE GROUP CONSISTING OF TANTALUM AND NIOBIUM, A THIN SHEET CONSISTING ESSENTIALLY OF TIN, A THIN SHEET OF AN ALLOY CONSISTING ESSENTIALLY OF ALUMINUM AND 1 TO 5% BY WEIGHT OF GALLIUM, A THIN WAFER OF AN N-TYPE SILICON, A THIN SHEET OF A SOLDER CONSISTING ESSENTIALLY OF TIN AND A DONOR IMPURITY, AND ANOTHER TRANSITION MEMBER FORMED OF ONE OF THE GROUP CONSISTING OF TANTALUM AND NIOBIUM MAINTAINING EACH OF THE COMPONENTS OF SAID STACK IN CONTACT WITH THE ADJACENT COMPONENTS; HEATING SAID STACK TO A TEMPERATURE OF BETWEEN 850* AND 1100*C.; MAINTAINING SAID STACK AT SAID TEMPERATURE FOR A PREDETERMINED TIME INTERVAL, AND THEREAFTER REDUCING THE TEMPERATURE OF SAID STACK. 